Cellular communication systems that communicate voice, data and signaling messages are being rapidly employed around the globe. For example, European Telecommunication Standard Institute (ETSI) has specified a Global Standard for Mobile Communication (GSM) that uses time division multiple access (TDMA) to communicate different types of information over radio frequency (RF) channels. In the U.S., Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), with the capability of transmitting voice and data to subscribers.
Generally, these types of communication systems cover a geographical area that is subdivided into communication cells, which together provide communication coverage to a service area, for example, an entire city. Each cell is served by one [or more] base station that communicate with mobile stations over downlink and uplink RF channels. The RF channels are subdivided into a number of time slots or logical channels during which data bits having various burst formats are communicated. GSM specification defines these formats as: normal burst (NB), access burst (AB), frequency correction bursts (FB), synchronization burst (SB), and dummy burst. Using the NB format, speech or text data bits are communicated during channels designated as traffic channels (TCH). Signaling data bits pertaining to call management within the system are communicated over control channels using one of the NB, SB or FB formats. Except for the dummy burst and FB, the remaining burst formats incorporate training sequences that are used by an equalizer to adjust the transmitted data bits at a receiving station.
In a GSM system, the control channels are grouped as broadcast channels (BCH), common control channels (CCCH), dedicated control channels, and a cell broadcast channel. The BCH are used for frequency correction, synchronization, and communicating cell specific information. As such, the BCH includes frequency correction channel (FCCH), synchronization channel (SCH), and broadcast control channel (BCCH). Data bits communicated over a FCCH burst, which uses the FB format, represent a sinus wave signal that serves to identify a RF channels carrying the BCH and CCCH and to enable the mobile stations to synchronize to these RF signal. Data bits communicated over a SCH burst, which uses the SB format, synchronize the mobile stations with the TDMA frame structure of a particular cell. Using a Base Station Identity Code (BSIC), the SCH data bits also identify a chosen cell as a GSM cell during a handover process. The CCCH are used for access and allocation of control channels. The CCCH include paging channel (FCH), access grant channel (AGCH), and random access channel (RACH). Among other things, the CCCH are used for paging a called mobile station, assigning a control channel, or initiating a call by a mobile station. According to the GSM specification, data bits communicated with the FB and SB formats are non-interleaved and are mapped on a single time-slot, i.e., time-slot 0, of a RF channel carrying the control channels used for call set-up procedure of a RF signal carrying the BCH and CCCH on a single burst. On the other hand, data bits communicated with the NB format, for example, the CCCH and TCH bursts, are interleaved and are mapped on several bursts.
Generally, the RF channels allocated to communication cells are patterned according to a reuse pattern that allows some of the spaced apart cells to use the same uplink and downlink RF channels. In this way, the reuse pattern of the system reduces the number of RF channels needed to cover the service area. It is, however, desirable to plan the cells using a tighter reuse pattern. The tighter reuse pattern is of particular importance, when communicating within a limited spectrum of, for example, 5-6 MHZ. Because the RF channels carrying the BCH and CCCH are a large portion of the total available spectrum, a tighter control channel reuse pattern would increase traffic capacity by allowing more RF channels to be allocated as the TCHs. A tighter control channel reuse pattern, however, may results in increased co-channel interference in other cells, thereby degrading system performance.
One conventional approach to improve the co-channel interference resistance teaches an antenna hopping technique. One such antenna hoping technique is disclosed, for example, in Hakan Olofsson et al., "Transmitter Diversity with Antenna Hopping for Wireless Communication Systems", in Proceeding of the IEEE VTC'97, in which different antennas are used for transmitting different bursts. In case a burst transmitted by one antenna is not received correctly, for example, due to severe fading, correct bursts transmitted by other antennas are used by interleaving and coding schemes to limit the bit error. With this antenna hopping technique, however, the transmitted data bits must be interleaved. Therefore, this technique is not suitable for communicating non-interleaved data bits, such as those communicated over the FCCH or SCH burst.
Another technique, known as delayed transmitter diversity, transmits the same burst via two or more uncorrelated paths, for example, from two spatially separated antennas. Under this technique, the bursts transmitted over one path are delayed (or have a small frequency offset) relative to the bursts transmitted over another path. Provided that a receiver equalizer can handle the delay (or offset), the transmitted bursts are demodulated based on the training sequences embedded on the bursts, thereby providing a diversity gain. However, the FCCH burst does not contain a training sequence. Consequently, this technique would not improve the FCCH performance, since it could not be equalized, due to its constant frequency structure.
Therefore, there is a need for improving performance, particularly control channel performance, in systems that employ a tight channel reuse pattern.